The way FreeBSD works, one cannot read from raw media using any block
size. You must use the actual block size on the media. With UDF and
ISO9660, this is not a problem. This might fail for FAT32 on DVD media.
If you find that the probe runs to completion, and still doesn't
detect the correct file system, you can dig around in the libvolume_id
code and try to fix this problem.